The discovery of the hepcidin peptide and characterization of its gene, HAMP,4 has led to the revision of previous models for the regulation of iron homeostasis and the realisation that the liver plays a key role in determining iron absorption from the gut and iron release from recycling and storage sites. Perhaps the most striking example has been to change the pathogenic model of HFE-related hereditary haemochromatosis from the crypt-programming model centered on the duodenal absorptive enterocyte to the hepcidin model centered on the hepatocyte.5,6 In summary, the hepcidin model proposes that the rate of iron efflux into the plasma depends primarily on the plasma level of hepcidin; when iron levels are high the synthesis of hepcidin increases and the release of iron from enterocytes and macrophages is diminished. Conversely when iron stores drop, the synthesis of hepcidin is down-regulated and these cells release more iron.
In order to describe the postulated major role of hepcidin it is necessary to understand the function of ferroportin, a protein first characterised in 2000. Ferroportin is the major iron export protein located on the cell surface of enterocytes, macrophages and hepatocytes, the main cells capable of releasing iron into plasma for transport by transferrin.7 
The major iron recycling pathway is centered on the degradation of senescent red cells by reticuloendothelial macrophages located in bone marrow, hepatic Kupffer cells and spleen. The exit of iron from these macrophages is controlled by ferroportin. The role of the hepatocyte is central to the action of ferroportin, because the hepatocyte is proposed to sense body iron status and either release or down-regulate hepcidin, which then interacts with ferroportin to modulate the release of cellular iron. Hepcidin directly binds to ferroportin and decreases its functional activity by causing it to be internalized from the cell surface and degraded.8 
Increased hepcidin synthesis is thought to mediate iron metabolism in two clinically important circumstances, shown schematically in FIG. 1. In individuals who do not harbour mutations causing haemochromatosis, the hepatocyte is thought to react to either an increase in iron saturation of transferrin or to increased iron stores in hepatocytes themselves, by inducing the synthesis of hepcidin by an as yet unknown mechanism. Thus the physiological response to iron overload under normal circumstances would be the hepcidin mediated shut down of iron absorption (enterocyte), recycling (macrophage) and storage (hepatocyte).
The synthesis and release of hepcidin is also rapidly mediated by bacterial lipopolysaccaride and cytokine release, especially interleukin-6. Thus the hepcidin gene is an acute-phase responsive gene which is overexpressed in response to inflammation. Cytokine mediated induction of hepcidin caused by inflammation or infection is now thought to be responsible for the anaemia of chronic disease, where iron is retained by the key cells that normally provide it, namely enterocytes, macrophages and hepatocytes. Retention of iron leads to the hallmark features of the anaemia of chronic disease, low transferrin saturation, iron-restricted erythropoeisis and mild to moderate anaemia.9 The nature of the hepcidin receptor is presently unknown, however an exciting future prospect may be the development of agents to block the receptor with the aim of treating the anaemia of chronic disease, a common often intractable clinical problem.
Down-regulation of hepcidin synthesis results in increased iron release, which arises in the two situations shown schematically in FIG. 2. The main causes of non-HFE haemochromatosis are mutations in either ferroportin, transferrin receptor 2, hepcidin or hemojuvelin genes. Classical HFE haemochromatosis, and all types of non-HFE haemochromatosis thus far studied with the exception of ferroportin related haemochromatosis, are characterised by inappropriate hepcidin deficiency. In these circumstances, hepatocytes become iron loaded, because their uptake of transferrin bound iron from the circulation is assumed to exceed that of ferroportin mediated export. Hepcidin deficiency causes increased ferroportin mediated iron export, resulting in increased enterocyte absorption of iron and perhaps quantitatively more important, enhanced export of recycled iron onto plasma transferrin by macrophages. Hepcidin is also suppressed in thalassaemic syndromes, both β thalassaemia major and intermedia and congenital dyserythropoetic anaemic type 1, where iron absorption is inappropriately stimulated despite the presence of massive iron overload.10 
As shown in FIG. 2, anaemia and hypoxia both trigger a decrease in hepcidin levels. These discoveries were made in animal models and need to be further studied to show they are applicable in humans. Two animal models of anaemia in mice were used to demonstrate a dramatic decrease in hepcidin synthesis where anaemia was provoked either by excessive bleeding or haemolysis.11 This is postulated to permit the rapid mobilisation of iron from macrophages and enterocytes necessary to allow for the increased erythropoietic activity triggered by erythropoietin release. The same study showed down-regulation of hepcidin synthesis can be triggered by hypoxia alone, and mice housed in hypobaric hypoxia chambers simulating an altitude of 5,500 m also showed a rapid decrease in hepcidin.
In summary, hepcidin provides a unifying hypothesis to explain the behaviour of iron in two diverse but common clinical conditions, the anaemia of chronic disease and both HFE and non-HFE haemochromatosis. The pathophysiology of hepcidin has been sufficiently elucidated to offer promise of therapeutic intervention in both of these situations. Administering either hepcidin or an agonist could treat haemochromatosis, where the secretion of hepcidin is abnormally low.
1. Park C H, Valore E V, Waring A J, Ganz T. Hepcidin, a urinary antimicrobial peptide synthesized in the liver. J Biol. Chem. 2001; 276:7806-10.
2. Pigeon C, Ilyin G, Courselaud B, et al. A new mouse liver-specific gene, encoding a protein homologous to human antimicrobial peptide hepcidin, is overexpressed during iron overload. J Biol. Chem. 2001; 276:7811-9.
3. Ganz T. Hepcidin, a key regulator of iron metabolism and mediator of anemia of inflammation. Blood. 2003; 102:783-8.
4. Roetto A, Papanikolaou G, Politou M, et al. Mutant antimicrobial peptide hepcidin is associated with severe juvenile hemochromatosis. Nat. Genet. 2003; 33:21-2.
5. Fleming R E. Advances in understanding the molecular basis for the regulation of dietary iron absorption. Curr Opin Gastroenterol. 2005; 21:201-6.
6. Pietrangelo A. Hereditary hemochromatosis—a new look at an old disease. N Engl J. Med. 2004; 350:2383-97.
7. Donovan A, Brownlie A, Zhou Y, et al. Positional cloning of zebrafish ferroportinl identifies a conserved vertebrate iron exporter. Nature. 2000; 403:776-81.
8. Nemeth E, Tuttle M S, Powelson J, et al. Hepcidin regulates cellular iron efflux by binding to ferroportin and inducing its internalization. Science. 2004; 306:2090-3.
9. Weiss G, Goodnough L T. Anemia of chronic disease. N Engl J. Med. 2005; 352:1011-23.
10. Papanikolaou G, Tzilianos M, Christakis J I, et al. Hepcidin in iron overload disorders. Blood. 2005; 105:4103-5.
11. Nicolas G, Chauvet C, Viatte L, et al. The gene encoding the iron regulatory peptide hepcidin is regulated by anemia, hypoxia, and inflammation. J Clin Invest. 2002; 110:1037-44.
The anemia of inflammation, commonly observed in patients with chronic infections, malignancy, trauma, and inflammatory disorders, is a well-known clinical entity. Until recently, little was understood about its pathogenesis. It now appears that the inflammatory cytokine IL-6 induces production of hepcidin, an iron-regulatory hormone that may be responsible for most or all of the features of this disorder. (Andrews N C. J Clin Invest. 2004 May 1; 113(9): 1251-1253). As such, down regulation of hepcidin in anemic patients will lead to a reduction in inflammation associated with such anemia.
Recently, double-stranded RNA molecules (dsRNA) have been shown to block gene expression in a highly conserved regulatory mechanism known as RNA interference (RNAi). WO 99/32619 (Fire et al.) discloses the use of a dsRNA of at least 25 nucleotides in length to inhibit the expression of genes in C. elegans. dsRNA has also been shown to degrade target RNA in other organisms, including plants (see, e.g., WO 99/53050, Waterhouse et al.; and WO 99/61631, Heifetz et al.), Drosophila (see, e.g., Yang, D., et al., Curr. Biol. (2000) 10:1191-1200), and mammals (see WO 00/44895, Limmer; and DE 101 00 586.5, Kreutzer et al.). This natural mechanism has now become the focus for the development of a new class of pharmaceutical agents for treating disorders that are caused by the aberrant or unwanted regulation of a gene.
Despite significant advances in the field of RNAi and advances in the treatment of pathological processes which can be mediated by down regulating HAMP gene expression, there remains a need for agents that can inhibit HAMP gene expression and that can treat diseases associated with HAMP gene expression such as anemia and other diseases associated with lowered iron levels.